Siemens SIMATIC S7 System Manual page 609
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Table 12- 74 Data types for the parameters
Parameter and type
MB_ADDR
IN
MB_HOLD_REG
IN
NDR
OUT
DR
OUT
ERROR
OUT
STATUS
OUT
Modbus communication function codes (1, 2, 4, 5, and 15) can read and write bits and words
directly in the input process image and output process image of the CPU. For these function
codes, the MB_HOLD_REG parameter must be defined as a data type larger than a byte.
The following table shows the example mapping of Modbus addresses to the process image
in the CPU.
Table 12- 75 Mapping of Modbus addresses to the process image
Codes
Function
01
Read bits
02
Read bits
04
Read words Input
05
Write bit
15
Write bits
Modbus communication function codes (3, 6, 16) use a Modbus holding register which can
be a M memory address range or a data block. The type of holding register is specified by
the MB_HOLD_REG parameter on the MB_SLAVE instruction.
Note
MB_HOLD_REG data block type
A Modbus holding register data block must allow both direct (absolute) and symbolic
addressing. When you create the data block the "Standard" access attribute must be
selected.
S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05
Data type
V1.0: USInt
V2.0: UInt
Variant
Bool
Bool
Bool
Word
Modbus functions
Data area
Address range
Output
1
Input
10001
30001
Output
1
Output
1
Description
The station address of the Modbus slave:
Standard addressing range (1 to 247)
Extended addressing range (0 to 65535)
Pointer to the Modbus Holding Register DB: The Modbus holding
register can be M memory or a data block.
New Data Ready:
0 – No new data
1 – Indicates that new data has been written by the Modbus
master
Data Read:
0 – No data read
1 – Indicates that data has been read by the Modbus master
The ERROR bit is TRUE for one scan, after the last request was
terminated with an error. If execution is terminated with an error,
then the error code value at the STATUS parameter is valid only
during the single scan where ERROR = TRUE.
Execution error code
Data area
to
8192
Output Process Image
to
18192
Input Process Image
to
30512
Input Process Image
to
8192
Output Process Image
to
8192
Output Process Image
105BCommunication processor
12.5 Modbus communication
S7-1200
CPU address
Q0.0 to Q1023.7
I0.0 to I1023.7
IW0 to IW1022
Q0.0 to Q1023.7
Q0.0 to Q1023.7
609
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